Size conversion

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A printed list of the exact metric sizes of imperial spanners, sockets & drill bits is handy. This is because some imperial tools make a good fit for metric nuts, bolts, etc, and vice versa (though of course many don't). A good example is 3/4", which is 19.05mm. A size conversion list can be stuck to the lids of socket, spanner & drill bit boxes.

Such a list

  • saves needing to find a spanner/socket/drill bit already in use
  • provides 2 of the same size tool for some sizes, which is occasionally very useful
  • enables old imperial tools to replace missing metric ones
  • avoids need to buy replacements sometimes
  • enables some metric tools to be used on old imperial equipment.
  • enables purchase & use of now very cheap old imperial tools
  • effectively expands your metric drill bit collection to include fractional mm sizes


This is definitely not a recommendation to use the sizes of socket & spanner that don't fit well. Doing so can damage nuts & spanners, and injure hands when the tool slips off.


Contents

Size tables

AF

Most imperial sockets & spanners are measured AF, across flats. These are marked with the size, and no following letters. Conversion is a straightforward inch to mm conversion.

  ____      
 /    \     AF means from
/      \    top to bottom
\      /    on this diagram
 \____/


64ths   Inch    mm      Fits
 
1/64            0.40
2/64  = 1/32    0.79
3/64            1.19
4/64  = 1/16    1.59    1.5
5/64            1.98    2
6/64  = 3/32    2.38
7/64            2.78
8/64  = 1/8     3.12    3
9/64            3.57
10/64 = 5/32    3.97    4
11/64           4.37
12/64 = 3/16    4.76
13/64           5.16
14/64 = 7/32    5.56    5.5
15/64           5.95
16/64 = 1/4     6.35
17/64           6.75
18/64 = 9/32    7.14    7
19/64           7.54
20/64 = 5/16    7.94    8
21/64           8.33
22/64 = 11/32   8.73
23/64           9.13
24/64 = 3/8     9.52    9.5
25/64           9.92    
26/64 = 13/32   10.32
27/64           10.72
28/64 = 7/16    11.11   11
29/64           11.51
30/64 = 15/32   11.91
31/64           12.30
32/64 = 1/2     12.7
33/64           13.1
34/64 = 17/32   13.49
35/64           13.89
36/64 = 9/16    14.29
37/64           14.68
38/64 = 19/32   15.08   15
39/64           15.48
40/64 = 5/8     15.87
41/64           16.27
42/64 = 21/32   16.67
43/64           17.07
44/64 = 11/16   17.46   17.5
45/64           17.86
46/64 = 23/32   18.26
47/64           18.65
48/64 = 3/4     19.05   19
49/64           19.45
50/64 = 25/32   19.84
51/64           20.24
52/64 = 13/16   20.64
53/64           21.03
54/64 = 29/32   21.43
55/64           21.83
56/64 = 7/8     22.22   22
57/64           22.62
58/64 = 29/32   23.02
59/64           23.42
60/64 = 15/16   23.81
61/64           24.21
62/64 = 31/32   24.61
63/64           25.0
64/64 = 1"      25.4mm  25

Sizes are only listed under the 'fits' column where the imperial size is in general use (or was). 29/32" may be a good metric fit, but no-one has such size tools.


Whitworth

The Whitworth system dates from 1841, and was the first non-proprietary bolt size system to be widely adopted. It describes bolt head sizes and bolt threads. There are 4 whitworth bolt designations, known as:

  • W (Whitworth)
  • BSW (BS Whitworth)
  • BSC (British Standard Cycle)
  • BSF (British Standard Fine thread)

BS and BSW sizes are often seen on old tools. In some countries they're still in common use, and Whitworth is still used here for some niche applications.

Whitworth sizes are not Across Flats measurements.


Whit     BSW/BSF   Inch AF   mm

1/16 W     -       0.256     6.90    
3/32 W     -       0.297     7.54    
1/8 W     3/16     0.340     8.64    
3/16 W    1/4      0.445     11.30    
1/4 W     5/16     0.525     13.34    (Camera tripod mounting)
5/16 W    3/8      0.600     15.24    
3/8 W     7/16     0.710     18.03    
7/16 W    1/2      0.820     20.83    
1/2 W     9/16     0.920     23.37    
9/16 W    5/8      1.010     25.65    
5/8 W     11/16    1.100     27.94    
11/16 W   3/4      1.200     30.48    
3/4 W     7/8      1.300     33.02    
13/16 W   15/16    1.390     35.31    
7/8 W     1.       1.480     37.59    
1. W      1.1/8    1.670     42.42    
1.1/8 W   1.1/4    1.860     47.24    
1.1/4 W   1.3/8    2.050     52.07    
1.3/8 W   1.1/2    2.220     56.39    
1.1/2 W   1.5/8    2.410     61.21    
1.5/8 W   1.3/4    2.580     65.35    
1.3/4 W   2.       2.760     70.10    
1.7/8 W    -        -        76.70    
          2.1/4    3.150     80.01    
2. W      
          2.1/2    3.550     90.17    
          2.3/4    3.890     98.81    
          3.       4.180     106.17    
          3.1/4    4.530     115.06    
          3.1/2    4.850     123.19    
          3.3/4    5.180     131.57    
          4.       5.550     140.97    
          4.1/2    6.380     162.05


BA

BA       Inch AF   mm

8 BA     0.152     3.86    
7 BA     0.172     4.37    
6 BA     0.193     4.90    
5 BA     0.220     5.59    
4 BA     0.248     6.30    
3 BA     0.282     7.16    
2 BA     0.324     8.23    
1 BA     0.365     9.27    
0 BA     0.413     10.49  = 7/32 BS


Number & Letter

File:Us drill sizes 2.gif A few DIYers also have number & letter sizes.

Gauge  Inch    mm

80	0.014	0.343	
79	0.015	0.368	
78	0.016	0.406	
77	0.018	0.457	
76	0.020	0.508	
75	0.021	0.533	
74	0.023	0.572	
73	0.024	0.610	
72	0.025	0.635	
71	0.026	0.660	
70	0.028	0.711	
69	0.029	0.742	
68	0.031	0.787	
67	0.032	0.813	
66	0.033	0.838	
65	0.035	0.889	
64	0.036	0.914	
63	0.037	0.940	
62	0.038	0.965	
61	0.039	0.991	
60	0.040	1.016	
59	0.041	1.041	
58	0.042	1.067	
57	0.043	1.092	
56	0.046	1.181	
55	0.052	1.321	
54	0.055	1.397	
Gauge	inches	mm	
53	0.059	1.511	
52	0.064	1.613	
51	0.067	1.702	
50	0.070	1.778	
49	0.073	1.854	
48	0.076	1.930	
47	0.079	1.994	
46	0.081	2.057	
45	0.082	2.083	
44	0.086	2.184	
43	0.089	2.261	
42	0.094	2.375	
41	0.096	2.438	
40	0.098	2.489	
39	0.099	2.527	
38	0.101	2.578	
37	0.104	2.642	
36	0.106	2.705	
35	0.110	2.794	
34	0.111	2.819	
33	0.113	2.870	
32	0.116	2.946	
31	0.120	3.048	
30	0.129	3.264	
29	0.136	3.454	
28	0.141	3.569	
27	0.144	3.658	
Gauge	inches	mm	
26	0.147	3.734	
25	0.149	3.797	
24	0.152	3.861	
23	0.154	3.912	
22	0.157	3.988	
21	0.159	4.039	
20	0.161	4.089	
19	0.166	4.216	
18	0.169	4.305	
17	0.173	4.394	
16	0.177	4.496	
15	0.180	4.572	
14	0.182	4.623	
13	0.185	4.699	
12	0.189	4.801	
11	0.191	4.851	
10	0.194	4.915	
9	0.196	4.978	
8	0.199	5.055	
7	0.201	5.105	
6	0.204	5.182	
5	0.206	5.220	
4	0.209	5.309	
3	0.213	5.410	
2	0.221	5.613	
1	0.228	5.791	
Gauge	inches	mm 

A	0.234	5.944	
B	0.238	6.045	
C	0.242	6.147	
D	0.246	6.248	
E	0.250	6.350	
F	0.257	6.528	
G	0.261	6.629	
H	0.266	6.756	
I	0.272	6.909	
J	0.277	7.036	
K	0.281	7.137	
L	0.290	7.366	
M	0.295	7.493	
N	0.302	7.671	
O	0.316	8.026	
P	0.323	8.204	
Q	0.332	8.433	
R	0.339	8.611	
S	0.348	8.839	
T	0.358	9.093	
U	0.368	9.347	
V	0.377	9.576	
W	0.386	9.804	
X	0.397	10.08	
Y	0.404	10.26	
Z	0.413	10.49	


What size is ok?

Sockets & spanners

It may be helpful to first explain that spanners and sockets aren't actually the size marked on them. Rather they're a size which will fit over nuts of the marked size. A slight degree of slack between tool and nut is required, otherwise spanners would often not fit on, and would often be an impractically stiff fit. So tools are already calculatedly oversize.

The question then is how much oversize is ok. And its not simple to answer, as it depends on several factors. There should be just a little free play between tool and nut. If the gap is too big, the tool can damage the nut and slip off under pressure. The acceptable gap also depends on the nut size, for example a difference of 1mm is ok on a 1" nut, but entirely non-functional on a 3mm nut. Steel nuts are more tolerant of oversize tools than brass. And the acceptable oversize also depends on the torque to be applied; at very low torque levels large amounts of oversize are ok. And finally it sometimes depends on whether the tool in use is hex, bi-hex or parallel jawed.

Perhaps someone will come up with an engineering table for safe gap limits some day. Until then I can only say that if there is any noticable amount of rotation of the tool relative to the nut, its no good, so the acceptable tolerances are fairly small.

The one exception to that is where the torque applied is very low. If a socket is to be rotated directly by hand, with no bar, then generally anything that even loosely fits will do.


Drill bits

With some jobs, drill bit size must be precise. But for most DIY this is far from true, and simply selecting the nearest size is usually absolutely fine. Thus more or less all imperial drill bit sizes are good for re-use in a metric world.

Size is critical when drilling a hole in metal that will be tapped, and close substitutes should never be used for this.


Labelling

If you want to use old tools as a regular part of a set of metric tools, remarking them in mm makes life easier.

Painting sizes on tends to wear off in use, it doesn't work well.

A good option is a die grinder fitted with any small abrasive tool, which can be used to write on metal like a pen.

Another option is to paint the whole outside of the spanner/socket/etc, and put a blob of that colour paint on the hole where it fits in the toolbox. (Each size uses a different colour paint.)


Issues

Some vehicles use a mixture of bolt systems by design. Land Rovers and some other old designs are known for this. Until 1955 Morris and MG used metric threaded bolts with their heads made to fit Whitworth sizes.


See Also

Personal tools